Pressure Constraints with Respect to Fault Seal Integrity and Reservoir Flooding in the Patchawarra Trough, Cooper Basin
Chandler, Emma S.
University of Adelaide
Knowledge of compartmentalisation in hydrocarbon reservoirs facilitates the planning of extended oil recovery (EOR) programs. Currently, little is known about the compartmentalisation of the Moorari and Fly Lake hydrocarbon fields in the Patchawarra Trough, Cooper Basin. The future of EOR and field development in these fields lies in the determination of the degree of compartmentalisation using a holistic approach to fault seal analysis.
Structural interpretation from a 3-D seismic survey identified three key faults in the Moorari Field with orientations that may affect their reactivation and sealing potential and thus the compartmentalisation of the field.
Three compartments in the Moorari field were identified using a saturation height function developed specifically for the Tirrawarra Sandstone. These compartments were located either side of the main north north-east trending fault with a third compartment to the south of the main field. The Fly Lake Field has four compartments interpreted by Adams (2002) applying the developed saturation height function.
Two most likely contemporary in situ stress tensors (reverse and strike slip fault regimes) with a σHmax orientation of 135o N were tested. Analysis suggests that the main north north-east trending fault in Moorari is less likely to be reactivated with respect to in situ stress whereas; the two cross cutting south east trending faults are more prone to reactivation.
Juxtaposition seals occur when faulting juxtaposes lithologies with different sealing capacities or capillary entry pressures. If sand on sand juxtaposition occurs then the sealing potential is reduced and another method of sealing is required. As the under and over lying lithologies in the Patchawarra trough contain a significant amount of shale clay smear was assumed to be a possible sealing method. The amount of clay entrained into the fault plane is estimated by shale gouge ratio (SGR).
Investigation of juxtaposition and shale gouge ratio (SGR) for the faults in the Moorari field concluded that; both cross cutting faults have small throws (average 16 m) and small SGR (0 – 0.20) and the main north north-east trending fault has large throw (average 100 m) and large SGR (0.3 – 0.6) as indicated on Triangle diagrams. Thus the main north north-east trending fault is more likely to be sealing than the cross cutting faults.
The Fly Lake Field contains the Tirrawarra sandstone formation and is within the same stress regime as the Moorari Field. Comparisons are thus made and similar methodologies applied to attempt to infer the potential for compartmentalisation of the Fly Lake Field. The Fly Lake Field contains three compartments inferred by the same saturation height function used in the Moorari Field